Cant get Fine, crisp displacements using displace modifier in 2.8
The displacement modifier is for non-adaptive displacement that needs to act on the mesh outside of just rendering it.

The displacement modifier only works on existing geometry and won't create new geometry. Adding a subdivision modifier above it can help, overall it is limited to coarser displacement of the mesh due to the performance limitations of generating lots of geometry this way.

The alternative is render time displacement, which can take advantage of the subdivision surface modifier's adaptive subdivision feature. This is the part of Cycles that was originally part of the experimental feature set. To make this work, you need to set the material's properties to use true displacement (or the 'both' option) and feed your displacement into the displacement socket of the material output in Cycles. 

Adaptive subdivision is performed at render time, as part of the rendering process, and tries to only generate geometry where it is actually needed. As part of these calculations are based on the camera position, it makes no sense to do this outside rendering.

for more info see https://docs.blender.org/manual/en/latest/render/materials/components/displacement.html